It is well known that the refractive index of a body of a fluoride glass, such as an optical fiber preform, depends not only on the composition of the glass but also on its thermal history. In effect, during the casting and drawing processes stresses or inhomogeneities may arise which give rise to local fluctuations in the refractive index, which fluctuations must be detected and eliminated.
The conventional devices for measuring the refractive index of samples of transparent material, which are based on measuring the limit angle (for instance Pulfrich refractometers) require sending a grazing beam onto the sample and analyzing the beam refracted by the sample. These devices cannot be employed to measure directly the refractive index of the core and the cladding of an optical fiber, since the size of the sample is too small to obtain a significant amount of refracted light. Using a Pulfrich refractometer for measurements on a glass sample whose composition is analogous to that of the preform, on the other hand, does not allow taking into account the thermal history of the glass, and in particular the fact that the cooling conditions of a wafer are different from those of a cylindrical body like the preform.
European patent EP-B 0 085 978, describes a method of determining the refractive index, which method can also be employed in the case of small samples, such as those obtained by cutting an optical fiber preform. According to the known method, the sample is placed on a support which can be rotated. A light beam comprising two monochromatic radiations is sent towards the sample with a first angle of incidence and the two radiations are caused to interfere upon leaving the sample, thereby creating a first beat. Subsequently, the support is rotated, the beam is directed against on the sample with a second angle of incidence and the two radiations are again caused to interfere upon leaving the sample, thereby creating a second beat. The refractive index is obtained from the phase differences between the two beats and a reference beat obtained by making the two radiations of the beam interfere at the end of a path external to the sample.
However, radiations which pass through the sample inevitably undergo multiple reflections inside it, and these bring about interference phenomena which result in a phase error limiting the accuracy of the measurement to such an extent that it is no longer possible to appreciate said fluctuations in the refractive index. Moreover, the measurement is quite sensitive to thermal expansion and to electrical drift.